The present invention pertains to cryogenic processes for recovering hydrogen and/or carbon monoxide from gas mixtures containing those and possibly other components, and in particular to such cryogenic processes which use hydrogen rejection membranes.
Syngas is a gaseous mixture consisting primarily of hydrogen (H2) and carbon monoxide (CO) which, depending upon the level of purity, may contain relatively small amounts of argon, nitrogen, methane and other trace hydrocarbon impurities. The primary uses of syngas are in the synthesis of methanol (requiring a hydrogen:carbon monoxide molar ratio of 2:1) and in reactions to produce oxo-alcohols (requiring a hydrogen:carbon monoxide molar ratio of at least 1:1). For many applications, it is necessary to control the relative proportions of hydrogen and carbon monoxide. This is achieved by, for example, cryogenically separating crude syngas into separate hydrogen-rich and carbon monoxide-rich streams and then combining those streams in the appropriate molar ratio to produce the required syngas composition. In addition to various syngas ratio adjustment applications, it is often desirable to extract and purify significant quantities of carbon monoxide and/or hydrogen from similar crude syngas feed streams. These carbon monoxide and/or hydrogen production processes can also be achieved through cryogenically separating the crude syngas into separate hydrogen-rich and carbon monoxide-rich streams before further purification and/or blending as appropriate. The level of impurities, especially methane and other hydrocarbons, in the crude syngas usually also is reduced during the cryogenic separation.
Existing technologies for the cryogenic processes that recover hydrogen and carbon monoxide use various methods of refrigeration that are relatively expensive and inefficient. Many of the difficulties with the existing technologies relate to the inherent nature of those methods of refrigeration. There are two main methods for providing refrigeration for processes with lower levels of H2 production when an external refrigerant is not available. The first method is to partially condense the H2xe2x80x94CO syngas feed and turbo-expand all or part of the H2-rich fraction that is not condensed from the syngas feed. This is often inefficient because of the large amount of refrigeration required to partially condense the feed stream when it contains significant quantities of H2. The second method is to use a membrane system to reject the excess H2 upstream of the cryogenic system and rely on the Joule-Thompson (J-T) refrigeration resulting from the lower pressure flashing of the cold feed stream. Although there are numerous variations on this method, the refrigeration from the feed stream J-T expansion is not always sufficient to operate the overall system.
There are several existing membrane integration schemes for cryogenic process cycles to produce carbon monoxide, hydrogen, and/or syngas. All of these processes have several features in common. All of the processes typically start with a crude syngas feed stream containing primarily hydrogen and carbon monoxide with lower levels of N2, Ar,CH4, and other trace hydrocarbon impurities. The syngas feed stream typically is passed over a semi-permeable membrane to remove varying levels of excess H2 while the H2-rich permeate typically is blended with fuel or taken as product at this point. The CO-enriched retentate stream typically is then cooled and partially condensed to partially separate most of the heavier components from the hydrogen. Any non-condensed remaining H2-rich stream may be washed with a condensed fluid, such as CH4, to remove further impurities in what commonly are known as CH4-wash cycles. In these wash cycles, the process refrigeration is most commonly provided by a pure carbon monoxide recycle system integrated with a carbon monoxide product compressor. In cycles without the wash step, commonly referred to as partial condensation cycles, the H2-rich stream is commonly expanded in a turbo-expander for refrigeration before it leaves the cryogenic part of the plant as a crude hydrogen product. This second crude hydrogen product is often further purified by pressure swing adsorption (PSA) and is sometimes compressed to final delivery pressure.
The remaining heavier liquid is then separated in one or more columns to remove the residual hydrogen, CH4, and optionally any other relevant impurities. The purified carbon monoxide is then rewarmed and typically leaves the cryogenic part of the plant as low pressure carbon monoxide product. This carbon monoxide stream is often compressed to final delivery pressure with part of the carbon monoxide stream sometimes compressed and returned to the cryogenic system to provide column reflux or as a heat pumping fluid.
There are numerous examples of this general purification scheme with various different methods of membrane integration disclosed in the patent literature. Some of these examples are discussed below.
U.S. Pat. No. 4,548,618 (Linde, et al.) discloses a membrane and cryogenic process integration for H2 removal and purification of light gases with a normal boiling point of less than 120xc2x0 K. Here, H2 is removed from the feed to the cryogenic system by the membrane. The H2-lean stream is then fed to the cyogenic system and is itself expanded to provide refrigeration for the process. The H2-rich permeate stream does not even enter the cyrogenic system and is discharged as byproduct.
U.S. Pat. No. 4,654,063 (Auvil, et al.) discloses integration of a membrane system with a non-membrane separation system (specifically including the case of a cryogenic system) to recover H2 from a feed gas mixture. Here, the membrane is used to remove H2 from the feed to the non-membrane separator and/or to take an H2-enriched stream from the non-membrane separator unit and remove H2 before recycling the subsequent H2-lean stream to the non-membrane separator. The H2-rich permeate streams in all of the embodiments are subsequently discharged as a product stream with optional compression.
EP 0359 629 (Gauthier, et al.) discloses generation of a H2/CO syngas from a feed with excess H2. This feed is passed through a permeator to adjust the H2/CO ratio by removing some H2 before at least a portion of the adjusted syngas is subsequently fed to a cryogenic system for the production of H2 and CO. The H2-rich permeate stream is directly discharged from the membrane as a byproduct.
JP 63-247582 (Tomisaka) discloses a process to separate CO from a predominantly CO and H2 feed which is passed to a membrane system immediately upstream of a cryogenic system to raise the concentration of CO in the gas fed to the cryogenic system. Here the refrigeration for the process is provided by a combination of J-T refrigeration from the feed stream and a supplemental liquid nitrogen (LIN) stream. The H2-rich permeate from the membrane is used for regeneration of adsorption based CO2 and H2O removal beds.
DE 43 25 513 (Fabian) describes a process for recovery of a high purity CO product stream and a H2 product stream using a membrane integrated with a cryogenic partial condensation cycle. An intermediate syngas stream is passed through a membrane to remove H2 before the stream is recycled to the cryogenic system to recover and purify the CO product. The H2-rich permeate is then discharged from the process as H2 product. The claimed benefit relative to a condensation cycle without a membrane is the elimination of the cold heat exchanger and H2 expansion refrigeration system. Fabian""s work is clearly focused on situations where there is sufficient J-T refrigeration in the feed stream to completely drive the overall separation process.
EP 0 968 959 (Billy) also discloses an integrated membrane and cryogenic process. Here the membrane rejects H2 from a CO/H2 stream produced at cryogenic conditions. The H2-depleted non-permeate stream is compressed and recycled back to the cryogenic system feed stream. The H2-rich permeate stream is not returned to the cryogenic process and is discharged as a byproduct. Refrigeration for the process is provided by expanding a separate H2-rich stream generated independently within the cryogenic system.
WO 99 67587 (O""Brien) discloses a membrane-enhanced cryogenic process for the production of a specific H2/CO ratio syngas stream. The membrane rejects a H2-rich permeate steam while the H2-depleted non-permeate stream is fed to the cryogenic system. The refrigeration for the process is provided by expanding a portion of the bottoms stream from a CH4 removal column. The H2-rich permeate stream is compressed and discharged from the system as a byproduct without entering the cryogenic portion of the system.
U.S. Pat. No. 6,161,397 (McNeil, et al.) also discloses an integrated membrane and cryogenic process. Here the cryogenic system produces multiple syngas streams and the membrane is part of a flow and H2/CO ratio control scheme. The H2-rich permeate stream is sent directly to fuel while the refrigeration for the process is provided by supplemental LIN.
It is desired to have a process which reduces the costs and improves the efficiency of a cryogenic condensation cycle hydrogen-carbon monoxide system producing carbon monoxide and optionally hydrogen and/or syngas co-products.
It is further desired to have a process for the production of hydrogen and carbon monoxide which overcomes the disadvantages and deficiencies of the prior art to provide better and more advantageous results.
The invention is a process and a system for recovering a first component and/or a second component from a multicomponent feed gas mixture containing the first component and the second component. There are several embodiments and variations of the process and the system.
A first embodiment of the process includes multiple steps. The first step is to pass the feed gas mixture through a membrane separation unit, thereby separating the feed gas mixture into a first stream enriched in the first component and a second stream lean in the first component. The second step is to cool the first stream. The third step is to expand the cooled first stream in a work extraction device, thereby generating a refrigeration supply for the process.
In a preferred embodiment, the first component is hydrogen and the second component is carbon monoxide. However, the process may be used for recovering one or more components from multicomponent feed gas mixtures containing components other than hydrogen and/or carbon monoxide.
There are several variations of the first embodiment of the process. In one variation, the membrane separation unit is a semi-permeable membrane adapted to reject the first component. In another variation, the work extraction device is a turbo-expander.
In another variation of the first embodiment of the process, the first stream is cooled by feeding the first stream to a heat exchanger. There are several variants of this variation. In one variant, the process includes the further step of withdrawing from the heat exchanger a product stream of the first component. In another variant, the process includes an additional step of feeding the second stream to the heat exchanger. In yet another variant, the process includes the additional step of withdrawing from the heat exchanger a product stream of the second component.
A first embodiment of the system includes four elements. The first element is a membrane separation unit. The second element is a means for passing the feed gas mixture through the membrane separation unit, thereby separating the feed gas mixture into a first stream enriched in the first component and a second stream lean in the first component. The third element is a means for cooling the first stream. The fourth element is a work extraction device adapted to expand the cooled first stream, thereby generating a refrigeration supply for the system.
In a preferred embodiment of the system, the first component is hydrogen and the second component is carbon monoxide. However, the system may be used for recovering one or more components from multicomponent feed gas mixtures containing components other than hydrogen and/or carbon monoxide.
There are several variations of the first embodiment of the system. In one variation, the membrane separation unit is a semi-permeable membrane adapted to reject the first component. In another variation, the means for cooling is a heat exchanger. In yet another variation, the work extraction device is a turbo-expander.
A second embodiment of the system is similar to the first embodiment, but includes an additional element. The additional element is at least one distillation column adapted to receive at least a portion of the second stream.